Industrial Control Basics – Pushbuttons

I always enjoy watching impatient people waiting for an elevator. They press the button, and if it doesn’t come within a few seconds they press it over and over again, as if this will hurry things up. In the end they must resign themselves to the fact that the elevator will operate in its own good time.

Pushbuttons, although simple in appearance like the big, red “Easy” button that’s featured in a certain business supply chain’s commercials, are actually complex behind the scenes. They perform important functions within the industrial control systems of a huge diversity of mechanized equipment.

Last week we introduced ladder diagrams, used to design and document industrial control systems, and we’ll now see how they depict the action of pushbuttons within two commonly used industrial settings, the “normally open” and the “normally closed.”

Figure 1

Figure 1(a) shows a pushbutton hooked up to an electric motor. When no one is pressing it a spring in the pushbutton forces the button to rest in the up position, allowing an air gap to exist in the electrical circuit between hot and neutral and preventing current from flowing. This type of switch is characterized as a “normally open” switch in industrial control terminology.

In Figure 1(b) someone depresses the button, compressing its spring and closing the air gap, which allows current to flow and the motor to operate

Figure 1(c) shows the ladder diagram version of 1(a).

Now let’s take a look at Figure 2 to see a different type of pushbutton, one that’s characterized as “normally closed.”

Figure 2

“Normally closed” refers to the fact that when no one is depressing the button, the normal operating position is for the air gap to be absent, allowing electrical current to flow and the motor to operate, as shown in Figure 2(a).

Figure 2(b) shows that an air gap is created when the button is depressed and the spring holding the mechanism into the normally closed position is forced down. This action interrupts electrical current and causes the motor to stop.

Figure 2(c) shows the simplified line drawing version of 2(a).

You can imagine how strained your finger would be if it had to press down on that button with any frequency or duration. Next time we’ll see how electrical relays work alongside pushbuttons to give index fingers a break.